High voltage half-bridge and full-bridge driver circuits are used in various applications such as motors, electronic ballasts, dual-inductor single-capacitor converters, and cold cathode fluorescent lamps (CCFL), and required to convert certain logic signals from low voltage to high voltage in order to control the high-voltage components. For example, in a half-bridge driver circuit as shown in FIG. 1, control signals Hin and Lin are used to switch a high side switch M1 and a low side switch M2 connected in series between a high voltage terminal VIN and a low voltage terminal GND, respectively. The direct-current (DC) supply voltage VIN is very high, maybe up to 600V or above, while the control signal Hin and Lin are generated by a logic circuit with reference to a low voltage power source VCC (e.g. 5V or 12V), and thus the control signal Hin for the high side switch M1 must be shifted from a low voltage level to a high voltage level. More particularly, in virtue to the operation of a low side circuit 10, a level shift circuit 12, and a high side circuit 14, a control signal S4 is derived from the control signal Hin for a high voltage driver 20 to generate a control signal UG for controlling the high side switch M1. On the other hand, the control signal Lin is directly provided to a low voltage driver 22 to generate a control signal LG for controlling the low side switch M2. During the high side switch M1 is on and the low side switch M2 is off, the voltage VS at the floating node 24 between the high side switch M1 and the low side switch M2 is equal to the DC supply voltage VIN. On the contrary, during the high side switch M1 is off and the low side switch M2 is on, the voltage VS at the floating node 24 is equal to the ground voltage GND. Assuming that VS=600V and GND=0V, when the switches M1 and M2 are switched, the across voltage thereof can be as high as 600V. Since the on resistance of either of the switches M1 and M2 is typically of mΩ order, a high current of kA order may be generated when the switches M1 and M2 are switched and likely damages the switch M1 or M2. Such conditions that the switches M1 and M2 suffer high across voltage are referred to “hard switching”.
To avoid hard switching, a VS offset detection circuit 16 is used to detect the voltage VS at the floating node 24, to generate a zero voltage switching (ZVS) signal S5 for the control signal generator 18 to determine whether a hard switching occurs. If so, a signal S6 is triggered for the low side circuit 10 to adjust the dead time of the switches M1 and M2. In addition, the signal S6 may be used to adjust the resonant frequency of the inductor L1 and the capacitor C1, or to turn off the driver circuit. However, in prior arts, for example U.S. Pat. No. 7,049,767, the VS offset detection circuit can detect the voltage VS only when the high side switch is off.